1. Field of the Invention
The present invention relates to a motor drive circuit.
2. Description of the Related Art
As a drive system of a brushless DC motor including drive coils with a plurality of phases (three-phase, for example), known is a PWM (Pulse Width Modulation) drive system in which the motor rotates at a rotation speed according to a duty ratio of a signal supplied to an output transistor. Also, a drive system called a linear drive system, PAM (Pulse Amplitude Modulation) drive system or the like is known, and with the linear drive system (PAM drive system), the motor rotates at a rotation speed according to an amplitude of a signal supplied to the output transistor.
Further, for the purpose of silencing of the brushless DC motor or the like, known is a soft switching drive system in which 180-degree energization is performed. For example, Japanese Patent Laid-Open Publication No. H5-161388 discloses a drive circuit of a brushless motor in which a rotation position of a rotor can also be detected by a sensorless drive system without using a position detection element such as a Hall element by switching between 120-degree energization and 180-degree energization.
As such, the brushless DC motor can be driven by the soft switching drive system, including the case of the sensorless drive system, thereby realizing silencing.
However, a drive current supplied to the drive coil of the motor has the phase thereof delayed with respect to a drive voltage applied to the drive coil, due to an influence of inductive reactance. Thus, at the time of phase switching in the soft switching drive system, a drive current may flow less easily or a counter torque component may be generated.
Also, the higher the rotation speed of the motor becomes, the greater the inductive reactance becomes, resulting in the remarkable phase delay of the drive current. Thus, even if the drive current is increased in order to rotate the motor at a high speed, the motor cannot be increased in rotation speed sufficiently.